Imaging tubes using transmissive photocathodes are well known in the art. Also well-known are optical devices such as telescopes which make use of lens systems in which a small central portion of an optical element is functionally different from the portions of the element around it. Reflective photocathodes are known in vacuum photocells and photomultipliers. Convergent electrostatic electron lenses are known in, e.g., night vision image tubes.